1. Field of the Invention
This invention relates to imaging apparatus and methods and, more particularly, to Terahertz or T-ray imaging systems and methods which utilize electromagnetic radiation in the Terahertz (THz) range as incident energy upon objects under examination.
2. Description of the Related Art
Terahertz (THz) or T-ray imaging systems have been developed to monitor, detect, and recognize the presence of certain materials or objects that are introduced to the field of view for the imaging system. These systems operate in a non-destructive and non-invasive manner. They have been suggested for applications including process control, materials inspection, biomedical imaging, fault detection in materials, material profiling, and packaging inspection, to name but a few. A description of one such imaging system is given in U.S. Pat. No. 5,623,145 issued to M. Nuss. In this description, the terms “Terahertz imaging”, “T-ray imaging”, and “THz imaging” are used interchangeably without any intended change or loss in meaning.
Recently, the need has increased to be able to detect and identify, both quickly and reliably, concealed explosives and other chemical and biological agents as these items increasingly become weapons of war and terrorism. Terahertz systems have been proposed for this task using their characteristic transmission or reflectivity spectra in the THz range (approximately 0.1 THz–10 THz). Explosives such as C-4, HMX, RDX, TNT, and naphthalene all have characteristic reflection and absorption spectra in the 0.1–2.0 THz range (corresponding wavelength range of 3 mm–0.15 mm). These materials are easily distinguishable from other non-hazardous materials such as human skin. In essence, explosives appear as different “colors” to the THz detector as compared to non-hazardous items. The same principles apply to THz detection and imaging of agents used in chemical and biological weapons.
By using THz spectroscopy and imaging, it is therefore possible to detect and identify explosives and other chemical and biological weapons, even when they are concealed in clothing, sealed packages, suitcases, or the like, because the THz radiation is readily transmitted through concealment materials such as plastics, clothing, luggage, paper products, walls, and other insulative (i.e., non-conductive or non-metallic) materials. Identification of these agents is accomplished by comparing the spectra measured for the reflected or transmitted THz signals with known calibration spectra. These spectra are substantially unique signatures that distinguish the agents from other objects such as keys, coins, human skin, and clothing. Since metals are relatively opaque to transmission of THz wavelengths and exhibit substantially constant reflection spectra, weapons such as handguns and knives are similarly identifiable by THz imaging techniques.
THz imaging systems proposed in the past have been based upon a single THz source and THz detector pair that are synchronously scanned transversely across the object being imaged in order to generate the two dimensional image. These systems consequently take a significant amount of time to acquire sufficient data and thereby generate the image of even a single small object. As a result, they are not suitable for applications that depend on real-time acquisition of THz images.
Current THz imaging is based on using a single THz source such as a short-pulsed laser or multiple continuous wave sources whose output signals are combined to produce the required THz difference frequency. One difficulty with extending either of these techniques to continuous wave THz imaging of coherent or incoherent THz radiation is that coherent continuous wave or short-pulsed laser sources are required. Moreover, a coherent phase relationship is needed between the laser sources that generate and that detect the THz signals. But imaging of an incoherent THz source is not possible through the use of any of these methods.
In U.S. Pat. No. 6,815,683 issued to J. Federici et al. on Nov. 9, 2004, THz imaging apparatus and methods are disclosed by the inventors herein for rapid and effective examination of a region of interest to detect the presence of certain compositions. This design and technique does not require a particular coherent or incoherent source of THz signals. Instead, it allows the flexibility to choose any source such as an electronic THz source, a laser-based THz illuminating source, or an incoherent ambient THz radiation which might be present, for example, from the sun. This system incorporates a substantially planar Terahertz (THz) Interferometric Imaging Arrays (TIIA) for remote-sensing applications. In utilizing such an array, it is generally assumed that the object of interest is sufficiently far enough away from the array that the incoming THz signal presents a substantially planar wave front.
The assumption about a planar wave front is reasonable for far field imaging. But it is does not hold for near field imaging. In near field applications, it should be understood that the wave front is curved and not planar. Therefore, the system described above would suffer inaccuracy in the detection and imaging of objects in the near field. Techniques have not been proposed to date to deal with the problem presented by curved wave fronts experienced when trying to apply the far-field THz imaging systems to the near-field imaging application.